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Caffeine works by plugging up your adenosine receptors. Adenosine is what tells your brain it's time to sleep. Your body still makes adenosine, but can't clean it up while the receptors are plugged. So once the caffeine wears off you've got a ton sloshing around and it takes effect.

There's also some evidence that the brains of habitual caffeine users grow extra receptors for adenosine and its friends, so you're more sensitive to them once you're off caffeine.